1. Field of the Invention
The present invention relates to a high-pressure fuel-feed pump employed in an internal combustion engine ("internal combustion engine" will hereinafter be termed "engine").
2. Description of the Related Art
A high-pressure fuel-feed pump of plunger-pump type according to the related art and employed in a fuel system of a gasoline engine is indicated in FIG. 26. A fuel pump 302 is housed in a fuel tank 301, and after fuel is pressurized at several hundred kpa by the fuel pump 302, it is sent under pressure to an intake port 304 on a fuel filter 303. A discharge port 305 on the fuel filter 303 is connected to an intake port 307 on a high-pressure fuel-feed pump 306. Drive force due to reciprocating motion of a piston 311 is conveyed to a camshaft 310 by an interconnecting mechanism composed of a connected rod 312, crankshaft 313, and belt 314, causing to rotate the camshaft 310 of the high-pressure fuel-feed pump 306. Fuel taken in from the intake port 307 is pressurized to a high pressure of from several Mpa to several tens of Mpa by the high-pressure fuel-feed pump 306, and is discharged via a discharge port 308 to a common rail 309. High-pressure fuel which has collected pressure in the common rail 309 is supplied via a branching passage 315 to injectors 317 provided in several cylinders of the engine. Accordingly, high-pressure fuel from the injectors 317 is sprayed directly into combustion chambers 316 within the cylinders.
Excess low-pressure fuel output from a bypass discharge port 318 of the high-pressure fuel-feed pump 306 is returned via a return passage 319 to the fuel tank 303. Within the common rail 309 is disposed a pressure sensor 320 to detect the pressure of fuel in the interior thereof, and the pressure signal detected by the pressure sensor 320 is input to an electronic control unit 321. The electronic control unit 321 controls the conductance timing of a solenoid valve 322 in accordance with the pressure signal detected by the pressure sensor 320 and with the speed and load of the engine and the like so that fuel-injection pressure assumes an optimal value, thereby controlling the amount of fuel discharged to the common rail 309. Additionally, the electronic control unit 321 outputs control signals to the injectors 317 to control the fuel-injection timing and spray time in accordance with the running state of the engine, being the speed, load, and the like of the engine.
However, with such a high-pressure fuel-feed pump according to the related art, a clearance of from several microns to several tens of microns between the cylinder inner peripheral wall and the plunger outer peripheral wall is required for plunger sliding. When fuel in the fuel pressurization chamber is pressurized during fuel injection, fuel leaks from the foregoing clearance, and the fuel which has lower viscosity than lubrication oil dilutes the lubrication oil of the engine and thereby the lubrication, cooling, and so on of the various areas of the engine become inadequate, and so engine reliability is caused to be reduced. Similarly, lubrication oil introduced for sliding-portion lubrication within the pump adheres to the plunger and forms an oil film, a fuel oil film on the cylinder inner peripheral wall and a lubrication-oil oil film on the plunger outer peripheral wall make mutual contact due to the sliding of the plunger, and oil leakage in which lubrication oil contaminates the fuel is thereby generated. Due to this oil leakage, lubrication oil within the engine is gradually consumed and thereby the lubrication, cooling, and so on of the various areas of the engine become inadequate, and so engine reliability is caused to be reduced, or the need to frequently perform replenishment of the lubrication oil may occur. Furthermore, the possibility also exists that lubrication oil in the fuel may become a cause of deposits on the nozzle and injector.
To solve this problem, reducing the amount of fuel leakage by installing a seal member on the cylinder inner wall to seal the outer peripheral wall of the plunger may be considered. However, in order to install a seal member on the cylinder inner peripheral wall, it is necessary to provide a space to house the seal member in the cylinder inner wall, and the problems exist whereby the machining steps of the cylinder increase and, along with this, the physical dimensions of the cylinder become larger. When installation space for the seal member is ensured without making the axial physical dimensions of the cylinder larger, the high-pressure seal length of the cylinder and plunger is shortened and seal efficiency drops. Additionally, the number of parts other than the seal member also increases, and so there exists the drawback of higher cost. Furthermore, if a sliding scratch with a depth of from several tenths to several microns is produced on the outer peripheral wall of the plunger because of low-viscosity gasoline, which has scant self-lubricating properties, the exists the problem wherein this sliding scratch contacts the seal member, the seal member is damaged, and seal performance may drop or leakage may occur along the sliding scratch and the amount of fuel may rise.
Increasing the hardness of the plunger by heat treatment, plating, or the like may be considered to prevent the occurrence of sliding scratching on the outer peripheral wall of the plunger, but when plunger hardness is increased, the cylinder becomes susceptible to wear. If the hardness of the cylinder is increased to prevent cylinder wear, ultimately it becomes impossible to prevent the occurrence of sliding scratching on the plunger. It is possible to reduce the occurrence of sliding scratching by introducing lubrication oil into the sliding portion of the plunger and cylinder, but there exists the problem of a sudden increase in oil leakage. Additionally, it is possible to maintain the seal performance of the seal member even if sliding scratching occurs on the plunger by enlarging the urging force of the seal member which contacts the plunger, but this is impractical because the speed of wear of the seal member is hastened.